Heretofore, there is a vehicle that mounts thereon a power train, which uses two kinds of power sources including a gasoline or diesel engine that is an internal combustion and an electric motor. Such a power train is called as a hybrid system. This electric motor is driven by electric power supplied from a high voltage battery mounted on the vehicle. For example, in the case where an AC motor is used as the electric motor, DC power outputted from the battery is converted to AC power by means of a circuit such as an inverter or the like, and the electric motor is driven by this AC power.
Since a battery in such a hybrid system is responsible for driving of a vehicle, reliability thereof is to be heightened. Further, since output characteristics of such a battery largely depends upon its usage state (such as a travel distance, a driving method of a driver, frequency of use of accessories such as an air conditioner and a car stereo system), it is difficult to determine a deteriorated state of the battery using only elapsed years from the beginning of use.
As the system that determines a deteriorated state of a battery and carries out control so as to inhibit deterioration of the battery in a predetermined case, a battery control apparatus for a car has been known (see Japanese Patent Application Publication No. 2007-323999 (hereinafter, referred to as “Patent Literature 1”), for example). In a battery control apparatus of Patent Literature 1, a rate of deterioration of a battery is calculated every predetermined time on the basis of voltage, an electric current and temperature of a battery, and a control to inhibit (or reduce) deterioration of the battery (change of a target SOC, restriction of a charge/discharge amount of the battery, and the like) is carried out in accordance with a comparison result between the calculated rate of deterioration and a reference rate of deterioration.
Namely, Patent Literature 1 discloses, in order to calculate a rate of deterioration, which is a deterioration level of a battery, that is, a charge state quantity (SOC: State of Charge), a resistance deterioration calculating method in which internal resistance (impedance) of the battery is detected and its growth rate is used, and a current capacity deterioration calculating method in which a current capacity is detected and a decreasing rate of its change rate is used. Further, Patent Literature 1 also discloses, as a method of detecting a current capacity, a method of calculating a remaining capacity SOCv on the basis of open voltage by an estimate value of battery open voltage and a method of calculating a remaining capacity SOCc on the basis of electric current accumulation.